1. Technical Field
Embodiments variously relate to switch circuitry and, particularly, but not exclusively, to use of such switch circuitry in readout circuit of a complementary metal-oxide semiconductor (“CMOS”) image sensor.
2. Background Art
Switch circuits are often found in semiconductor devices such as CMOS image sensors. A popular form of switch circuit is a sample and hold circuit, which can operate in many applications such as data conversion and switch-capacitor filtering. FIG. 1 shows a circuit diagram of conventional switch circuit 100. Switch circuit 100 includes n-channel MOS (NMOS) transistor 110 and capacitor 120 connected in series with one another between an input port and an output port. When NMOS transistor 110 is switched on, by asserting control signal CTRL 130, the switch circuit 100 is closed to allow charging of capacitor 120. When CTRL 130 is de-asserted, switch circuit 100 is open. One problem with switch circuit 100—as with similar switches—is charge injection that takes place as a result of CTRL 130 being de-asserted. Charge injection redistributes charge in the channel of NMOS transistor 110, which tends to introduce noise to the output terminal of NMOS transistor 110.
A conventional technique for addressing charge injection is to operate switch circuit 100 with a boosted version of control signal CTRL 130. However, applying a boosted control voltage to the gate of NMOS transistor 110 over time results in accelerated device failure. Another technique for addressing charge injection is to connect a dummy circuit, such as dummy transistor 150, between NMOS transistor 110 and capacitor 120. Dummy transistor 150, responsive to control signal CTRLB 135, operates as a MOS capacitor to reduce charge injection from NMOS transistor 110 to capacitor 120. CTRLB 135 is substantially inverse of CTRL 130.
As integrated circuit fabrication techniques continue to improve, successive generations of smaller and/or faster semiconductor devices are increasingly sensitive to sources of signal noise such as charge injection.